1. The Field of the Invention
The invention relates to a generalized frequency division multicarrier transmission system and corresponding method to mitigate intercarrier and intersymbol interference. In particular the invention relates to an orthogonal multicarrier transmission system using an Offset—Quadrature Amplitude Modulation (QAM) modulation wherein a frequency shift of half a subcarrier bandwidth is used.
2. The Relevant Technology
Current 4G LTE-(Advanced) systems are based on OFDM that provides intersymbol interference (ISI)-free and intercarrier interference (ICI)-free transmissions in ideal AWGN channels, i.e. in channels where only white Gaussian noise is added to a transmit signal. However, if the channel is time-frequency dispersive such as a real channel of a cellular communication system, good time-frequency localization of the transmit signal is required to cope with asynchronicities. Furthermore, low out-of-band radiation is required to enhance spectral agility and aggregation of carriers.
One way to mitigate interference in transmissions is to use so-called Offset-QAM (OQAM) instead of conventional QAM modulation in a multicarrier system, for example OFDM/OQAM. In Offset-QAM modulation complex valued data symbols ck,m are transmitted on k=1 . . . K subcarriers, where the real and imaginary portion of a symbol are offset by half a symbol duration T, i.e. by ½T. To mitigate inter channel interference each symbol is pulse-shaped with a symmetric, real-valued pulse shaping filter.
The application of conventional Offset-QAM in multicarrier systems requires the use of pulse shaping filters, wherein these have to be symmetric in both the time and the frequency domain and furthermore have to be half-Nyquist filters. Furthermore, in conventional Offset-QAM systems, a phase shift of ½π is required between subcarriers and subsequent symbols, i.e. adjacent sub-carriers are shifted in phase by ½π against each other. Accordingly, this kind of Offset-QAM can be called time-shifted offset-QAM, in short time-shifted OQAM, since the imaginary part of a symbol is shifted in time by half a symbol duration.
In the above described time-shifted Offset-QAM systems complex-valued data symbols ck,m are transmitted on a plurality of k subcarriers, wherein the real and imaginary part are offset by ½T as described above, with T being the symbol duration. Each symbol is pulse-shaped by a prototype filter g(t), which can also be a non-symmetric conjugate-root filter.
Besides for OFDM systems, Offset-QAM has been proposed for GFDM systems. Similar to OFDM, GFDM is a multicarrier transmission system, but wherein circular convolution is applied instead of linear. Hence, a GFDM transmit signal exhibits a block structure, i.e. a plurality of subcarriers each conveying a plurality of symbols, wherein consecutive blocks can be decoupled/separated by appending a cyclic prefix to ease equalization at the receiver side. Hence, GFDM provides good time-frequency localization (TFL).
GFDM can use a conventional QAM modulation to achieve maximal efficiency for some known configurations, but the Balian-Low theorem (BLT) shows that good time-frequency localization can result in distorted reconstruction of complex-valued symbols sent at Nyquist rate.
To mitigate this problem the use of Offset-QAM has been proposed as solution, in particular with an above mentioned non-symmetric conjugate-root pulse-shaping filter. However, the use of time shifting in conventional Offset-QAM reduces the effect of using guard sub-symbols to achieve low out-of-band radiation.
Hence there is a need to provide an improved GFDM system that at least mitigates some of the above mentioned problems.